Abstract
Quasi-crystal structures are conventionally built following deterministic generation rules although they do not present a full spatial periodicity. If used as laser resonators, they open up intriguing design possibilities that are simply not possible in conventional periodic photonic crystals: the distinction between symmetric (vertically radiative but low quality factor Q) and anti-symmetric (non-radiative, high Q) modes is indeed here fully overcome, offering a concrete perspective of highly efficient vertical emitting resonators. We here exploit electrically pumped terahertz quantum cascade heterostructures to devise two-dimensional seven-fold quasi-crystal resonators, exploiting rotational order or irregularly distributed defects. By lithographically tuning the lattice quasi-periodicity and/or the hole radius of the imprinted patterns, efficient multimode surface emission with a rich sequence of spectral lines distributed over a 2.9–3.4 THz bandwidth was reached. We demonstrated multicolor emission with 67 mW of peak optical power, slope efficiencies up to ≈70 mW/A, 0.14% wall plug efficiencies and beam profile results of the rich quasi-crystal Fourier spectrum that, in the case of larger rotational order, can reach very low divergence.
Highlights
Two-dimensional (2D) photonic structures have been widely investigated in recent years, since they can be intriguingly engineered to accurately control the optical properties of passive or active devices such as optical fibers [1], waveguides [2] or lasers [3,4,5] over a broad frequency range, from the visible to the far-infrared
Laser action has been demonstrated at mid-IR [6] and terahertz (THz) frequencies [7] providing a fascinating solution for the achievement of simultaneous spectral and spatial mode engineering [8,9,10]
In a photonic crystal quantum cascade laser (QCL), operation is normally achieved on modes at the edges of photonic bandgaps [11] or on the localized states formed by suitably designed defects [12] within the periodic photonic lattice
Summary
Two-dimensional (2D) photonic structures have been widely investigated in recent years, since they can be intriguingly engineered to accurately control the optical properties of passive or active devices such as optical fibers [1], waveguides [2] or lasers [3,4,5] over a broad frequency range, from the visible to the far-infrared. In a photonic crystal QCL, operation is normally achieved on modes at the edges of photonic bandgaps [11] or on the localized states formed by suitably designed defects [12] within the periodic photonic lattice. This implies that efficient vertical out-coupling is typically hindered by the symmetry of the lasing modes, which usually leads to power cancellation in the far-field. This issue can be circumvented by using quasi-crystal patterns ([13,14]), in which the distribution of the dielectric scatterers deviates from periodicity while still being governed by a deterministic generation rule
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